Int. J. Electrochem. Sci., 14 (2019) 7717 – 7728, doi: 10.20964/2019.08.01
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.orgProperties of Jet electrodeposition Nickel Coating on TC4 Alloy
Prepared by Selective Laser Melting
Xiao Wang1, Zhanwen Wang1, Lida Shen*, Mingyang Xu, Kai Wang, Zongjun Tian National Key Laboratory of Science and Technology on Helicopter Transmission,
Nanjing University of Aeronautics and Astronautics, Yu Dao Street, 210016 Nanjing, China
1These authors contributed equally to this work *E-mail: ldshen@nuaa.edu.cn
Received: 21 January 2019 / Accepted: 18 March 2019 / Published: 30 June 2019
In order to solve the problems of poor surface wear resistance and low hardness of titanium alloy materials formed by selective laser melting (SLM) technology, a new method of preparing bright nickel protective coating by jet electrodeposition was proposed in this paper. The surface morphology of the coatings was characterized by field emission scanning electron microscopy. The microhardness tester, scratch tester, and friction and wear tester were used to analyze the adhesion, hardness, and wear resistance of the coatings. The results indicated that the current density has an important effect on the properties of the coatings; as the current density increases, the surface flatness gradually increases and then decreases. When the current density is 120 A/dm2, the microhardness of the coatings reaches 548 HV, and the adhesion strength also reaches 30.4 N. Compared with the surface of titanium alloy, the surface of the coating has better wear resistance, and the surface is more complete after friction and wear test. The wear mechanism is transformed from microscopic grinding to smearing.
Keywords: SLM; jet electrodeposition; adhesion strength; wear resistance
1. INTRODUCTION
energy source, directly forming the parts. This technology is suitable for many fields [8]. Many scholars have carried out extensive studies and achieved numerous results [9-10]. However, titanium alloys formed by SLM technology have poor wear resistance, are prone to contact corrosion, and have poor electrical and thermal conductivity, seriously restricting their application [11-12]. Therefore, it is necessary to apply some surface treatments on the titanium alloy in order to expand its application scope and improve its service life.
The surface treatment methods of titanium alloys include anodic oxidation [13], micro-arc oxidation [14], electroplating [4, 15], and electroless plating [16]. Among them, the method of anodic oxidation and micro-arc oxidation forms a dense oxide film on the surface of the titanium alloy, protecting the titanium alloy. However, because of its thin film and poor wear resistance, its performance is seriously affected. Electroplating deposits a coating on the surface of the titanium alloy by the principle of cathode deposition, which has the characteristics of uniformity and low porosity. Therefore, the properties of titanium and its alloys can be improved by deposition protection. [17-21]. There have been many scholars who have previously explored this area. Xia et al. show that NiCrAlY coating can significantly improve the ablation resistance of TC6 titanium alloy [17]. Peng et al. found that the deposition of Ta-W coating has a very good effect on protecting titanium and its alloys in an ablation atmosphere [22]. Yu et al. obtained a copper plating layer with good adhesion strength by activating and hydrogenating the titanium alloy. It was found that the electroplated copper improved the conductivity of the titanium alloy matrix and weakened its galvanic corrosion tendency [23].
Owing to the flexibility, being able to adapt to different surfaces, and having a large limiting current density of jet electrodeposition, we attempt to deposit a nickel coating on the titanium alloy formed by SLM using jet electrodeposition. In this paper, we will research the effects of different current densities on the surface morphology, adhesion, hardness, and wear resistance of the coatings.
2. EXPERIMENTAL
2.1 Experimental device
Figure 1. Experimental device
2.2 Experimental process and parameters
Titanium alloy formed by SLM technology was selected as the experimental substrate. Its size was15×15×3 mm, in which the laser scanning rate was 840 mm/s, the power was 220 W, and TC4 powder was used with an average particle size of 30 μm. The surface of titanium alloy was manually polished with 400#, 800#, 1000# metallographic sandpaper, followed by ultrasonic cleaning with deionized water, and alcohol, membrane removal, ultrasonic cleaning with deionized water, and nickel plating.
Table 1. Electroplating bath composition and processing parameters
Composition and operating conditions Content
Solution
NiSO4.6H2O (g/L) 280
NiCl2.6H2O (g/L) 40
H3BO4 (g/L) 40
C7H5O3NS (g/L) 5.0
PH 4.0±0.1
Operating conditions
Nozzle size(mm) 20×1
Anode-cathode distance(mm) 2.0
Rate of flow (L/h) 200
Scanning speed (mm/s) 5
Temperature (◦C) 50±1.0
The composition and processing parameters of the plating solution are shown in Table 1. The reagents in all of the solutions are analytically pure. The role of NiSO4˙6H2O and NiCl2.6H2O in the
solution is to provide nickel ions for the deposition process, and the role of C7H5O3NS is to refine the
grains [24]. The addition of H3BO4 not only adjusts the pH of the solution, but also enhances the
[image:3.596.80.518.475.656.2]
Table 2. Samples with different process parameters
Sample number Working current(A/dm2) Plating time (min)
1 60 40
2 80 30
3 100 24
4 120 20
5 140 17
2.3 Properties characterization
Table 3 shows the test equipment for analyzing the surface morphology, microhardness and protective properties of coatings, as well as their models and parameters.
Table 3. Testing equipment
Instruments (Type) Parameters Characterization
Scanning electron microscopy
(S-4800)
5.0kV ×500 or 5.0kV ×50
or 5.0kV ×2000 Surface morphology Micro-hardness tester
(HXS-1000A) Load 2N, loading time 10s Micro-hardness Scratch tester
(WS-2005) 40N, 3mm,Friction force Adhesion strength Friction and wear
(HSR-2M)
Load 10N, loading time
240s Wear resistance
3. RESULTS AND DISCUSSION
3.1 Surface morphologies
[image:4.596.84.501.320.463.2][image:5.596.122.476.409.693.2]
the surface of the deposited layer and make the surface have a small amount of ravine stripes [26]. When the current density increases to 100 A/dm2, a slightly uneven position on the surface of the titanium alloy will quickly form a bulge because of the high current density, and as the deposition proceeds, the bulge will become increasingly more obvious. Compared with Fig. 2a, the number of cellular bumps on the surface of the coating noticeably increases, as shown in Fig. 2b. This is attributable to the increase of nucleation points on the cathode surface caused by the high current. As the current density continues to increase to 120 A/dm2, the nucleation points on the surface of the substrate continue to grow in number. Further, the curvature of the cellular processes is larger, the equipotential lines are denser, and the electric field intensity significantly increases. Owing to the electric field being shielded by large protrusions, many small protrusions gradually stop growing during continuous deposition, and the growth rate of large protrusions becomes faster and faster, making them collide with each other and submerge small protrusions. The large protrusions that collide with each other improve the smoothness of the coating to a certain extent. As shown in Fig. 2c, the morphology of the coating is obviously better that observed in Fig. 2a and Fig. 2b. As shown in Fig. 2d, when the current density is increased to 140 A/dm2, the surface of the plating layer has defects such as large protrusions and pits. On the one hand, this is caused by the current density being too high and the deposition rate being too fast. On the other hand, nickel ions in the vicinity of the cathode are rapidly consumed, and a serious shortage of nickel ions causes hydrogen ions in the solution to be reduced, thereby increasing plating defects [27].
3.2 Adhesion strength
The cross-section morphology and scratches of the samples are shown in Fig. 3. The samples are cut by a wire-cutting machine, then gently polished and washed with alcohol. Fig. 3a depicts cross-section morphology with current density of 60 A/dm2. It can clearly be seen that there is a slight gap between the coating and the substrate, adhesion effect is not ideal. Fig. 3c presents the cross-section morphology of 120 A/dm2 current density. In the figure, the coating and the substrate adhesion are very close without any obvious cracks, and even some Ni are embedded in the substrate. It can be inferred that the coating with current density of 120 A/dm2 has stronger adhesion strength than that with current density of 60 A/dm2.
In this test, the scratch method was used to quantitatively analyze the bonding force between the coating and substrate. When the frictional force significantly fluctuates, this denotes that peeling occurs between the coating and the substrate, and the value of the load applied at this time is used as a basis for evaluation. Fig. 3b and Fig. 3d show the indentation of current density of 60 A/dm2 and 120
[image:6.596.131.466.417.690.2]A/dm2, respectively. It can be seen that the indentation of Fig. 3b is obviously fractured and falls off. The indentation of Fig. 3d has no cracks until the end, which is consistent with the results of cross-section morphology of Ni coatings on substrate. The adhesion strength of the coating is shown in Fig. 4. When the current density increases, the adhesion of the coatings first increases. When the current density is 120 A/dm2, the adhesion strength of the coatings reaches the maximum of 30.4 N, and then decreases with the increase of the current density.
Figure 4. Coating adhesion strength prepared at different current densities
Since the surface of titanium alloy is only polished by hand, there are inevitably protrusions and depressions, as well as gully-like stripes, on the surface. When the current density is low, the surface of the coating has fewer nucleation points. Further, because of the "edge effect", nickel ions will preferentially deposit on the protruding part of the substrate surface. The pitting and gully-like stripes on the substrate surface cause a slight gap between the coating and the substrate, seriously affecting the adhesion strength between the substrate and the coating. However, when the current density is increased, the surface nucleation point is increased, the plating defects are reduced, the uniformity and density of the plating layer are improved, and the adhesion strength is greatly improved. When the current density is too large, the hydrogen evolution reaction will increase because of the lack of nickel ions, and the gas and impurities generated by the reaction will remain between the substrate and the coating, easily causing cracking of the coating and reducing the adhesion strength.
3.3 Microhardness.
Fig. 5 shows the microhardness of the nickel coatings prepared at different current densities on titanium alloy. The microhardness of nickel coating first increases and then decreases with the increase of current density. The maximum microhardness is 548 HV, and the corresponding current density is 120 A/dm2. Then, with the increase of current density, the microhardness of the coating gradually decreases. The dashed line in the figure shows the hardness of the titanium alloy formed by SLM technology, which is 372.1 HV. It can be seen that its hardness is low. One reason for this is that some of the powder particles in the SLM process are not fully melted, resulting in voids [28]. Second, the stability of laser energy output and the error of powder layer thickness make the forming quality of each layer slightly different, thus affecting the overall hardness of titanium alloy [29].
[image:7.596.109.490.70.247.2]
morphologies of the coatings prepared with different current densities are different. When the current density is 120 A/dm2, the surface defects of the coating are few, and the density is high, so its hardness reaches the maximum. Continuous increase of current density will increase the number of surface defects and larger cellular processes, significantly affecting the microhardness of the coatings.
Figure 5. Microhardness of the coating under different current density
3.4 Wear resistance
Fig. 6 shows the surface morphology of different samples after wear test. Fig. 6a is the morphology of titanium alloy after wear test. It can be observed from the figure that the surface is almost entirely all deep furrows. It can be considered that the wear mechanism is mainly microscopic grinding, indicating that the surface hardness of pure titanium is low and the wear resistance is poor. According to the hardness analysis above, the hardness of titanium alloy formed by SLM technology is not high and the strength is low. Most materials are easy to fall off under the action of micro-cutting and form abrasive debris, which attaches to the friction surface. This, further aggravates the wear, and makes the volume of wear increase, thus forming a deep plough-groove-like wear mark on the surface; Fig. 6b is its partial enlargement morphology. Fig. 6c is the surface of the coating with current density of 60 A/dm2. From the morphology of the coatings after wear, there are obvious cracks, peeling, and
[image:8.596.174.424.155.360.2]
nucleation speed of the grains is slow, resulting in the lower compactness, coverage, adhesion, and hardness of the coating, and worsening the wear resistance. When the current density is high, the deposition rate of cathode surface is too fast, and defects such as nodulation, pitting, and burning are easily formed on the coating surface [33]. These factors are not conducive to improving the wear resistance of the coating surface. Therefore, when the current density is 120 A/dm2, the wear resistance
[image:9.596.143.456.201.553.2]is the best.
Figure 6. Surface morphology after coating wear (a, b. titanium alloy surface; c, d. 60 A/dm2; e, f. 120 A/dm2)
increase in friction coefficient, and finally stabilizing in a stable region, i.e., the stable wear stage [34]. The friction coefficient of the coating with different current densities has a certain fluctuation. From Figure 7, the friction coefficient of the sample with current density 120 A/dm2 is the smallest, about 0.65-0.7, followed by 140 A/dm2 and 60 A/dm2, about 0.8-0.85. When the experimental conditions are fixed, the surface of the coating has more rough peaks when the friction coefficient is large, while the surface roughness peaks are less when the friction coefficient is small [35].
Figure 7. Relationship between friction coefficient of coatings and time
4. CONCLUSIONS
(1) Smooth and compact nickel coating can be deposited on the surface of TC4 formed by SLM technology by using appropriate current density (120 A/dm2). When the current density is too low or
too high, there will be many bumps or defects on the surface of the nickel coating.
(2) With the increase of current density, the adhesion and hardness of the coating increases first and then decreases. When the current density is 120 A/dm2, the coating adhesion reaches the highest at 30.4 N, and the microhardness increases from 372 HV on TC4 surface to 548 HV.
(3) The wear mechanism of pure titanium substrate is microscopic grinding, with deep furrows on the surface. When the current density increases from 60 A/dm2 to 120 A/dm2, the wear mechanism
[image:10.596.128.470.188.471.2]
ACKNOWLEDGMENTS
The project is supported by the National Natural Science Foundation of China (Grant No. 51475235, No. 51105204 and No.U1537105). We also extend our sincere thanks to all who contributed in the preparation of these instructions.
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